1. Technical Field
The present disclosure relates to an OFDM system. More specifically, the present disclosure relates to a method for reducing a PAPR of an OFDM signal and an OFDM transmitter using the same.
2. Description of the Related Art
The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
OFDM systems transmit modulation signals using a number of orthogonal sub-carriers and transmit data by dividing the entire transmission band into a number of narrow band orthogonal sub-channels. Since the OFDM orthogonal characteristics permit sub-channels to overlap and, thus, increase the spectral efficiency. In addition, OFDM systems are robust to frequency selective fading compared to a system using a single carrier.
In most wireless communications systems including OFDM systems, a high power amplifier (HPA) is employed at the transmission stage in order to obtain sufficient transmission power. In order to obtain the maximum output power from such a HPA, the operating point is usually set to be close to the saturation region. This leads to non-linear distortion and in turn in-band distortion and out-of-band radiation, thereby significantly degrading the system performance. Such non-linear characteristics of a HPA are very sensitive to a change in the amplitude of a transmission signal. Thus, in an OFDM system that transmits combined modulated signals with a number of sub-carriers, the amplitude of the transmission signal is greatly changed. As a result, the peak-to-average power ratio (PAPR) of the transmission signal is very large compared to a system using a single carrier.
Recently, a lot of research has been conducted to reduce the PAPR of an OFDM signal. Among others, the peak clipping scheme is most commonly used in currently available systems. The peak clipping scheme reduces the PAPR by setting a threshold value and simply clipping a signal above the threshold value on the time axis. This scheme can be simply implemented and can achieve a desirable PAPR performance. However, the signal waveform is not smooth but is sharply cut, so that distortion occurs in a signal to be transmitted, thereby degrading the error vector magnitude (EVM). In addition, the amount of emission into adjacent bands, i.e., the adjacent channel leakage ratio (ACLR) also becomes larger.
As another scheme, the peak windowing scheme may be employed. According to the peak windowing scheme, when a peak signal above a threshold value occurs, a window is applied the position to reduce the peak signal as well as signals around the peak signal, so that the signal is smoothly cut. As the signal is smoothly cut, the peak windowing scheme exhibits better ACLR than the peak clipping scheme. However, the peak windowing scheme fails to exhibit good EVM since signals adjacent to the peak signal are also affected. Further, if the distance between two peak signals above the threshold value is smaller than the width of the window, signals are reduced to be smaller than the threshold value, and accordingly the overall performance degrades.